1. Field of the Invention
The present invention relates in general to range imaging systems and more particularly to a method and a system for measuring a relative position and orientation of range cameras using a movement of an object within a scene.
2. Related Art
Range imaging systems are used in a variety of applications to determine the three-dimensional (3-D) characteristics of a scene (a scene is an environment of interest). By way of example, these applications include 3-D scene reconstruction, 3-D object recognition, robot navigation, terrain mapping and object tracking. An important component of a range imaging system is a range camera. A range camera is a device that is used to measure a 3-D structure of a scene by providing range (or depth) information as measured from a plane on the camera. Thus, while a black and white camera provides a grayscale intensity of each pixel and a color camera provides a color of each pixel, a range camera provides a range (or distance to the 3-D scene) of each pixel. Range cameras use a variety of techniques to measure range including lasers, projected light patterns and stereo vision.
For some applications (such as tracking persons within a scene) the range imaging system may include more than one range camera because a single range camera may not have a sufficiently large field of view to monitor the entire scene. In order for multiple range cameras to work together, however, the cameras must be calibrated to determine a position and an orientation of each camera relative to one of the cameras (known as a relative pose). This calibration of multiple cameras enables the ranging system to convert 3-D measurements obtained from each camera into a common coordinate frame. For example, a path of a person in a scene may be measured by each camera in its local coordinate frame and converted to a common coordinate frame (such as a room-based coordinate system).
Several types of manual calibration techniques are used to calibrate the range cameras. One type of calibration technique uses a three-dimensional calibration chart to determine the relative position of each camera. This technique, however, is difficult to use and time-consuming because it requires that the calibration chart be positioned correctly within a scene.
Another type of calibration technique requires a user to monitor a scene and determine a plurality of reference points in the scene until the relative position of each camera can be determined. For example, a user references a number of common points in a scene (within each camera""s field of view) and, if enough of these common points are found, the relative pose of the cameras may be determined. One disadvantage of this technique, however, is that it is difficult to implement in a consumer-based product because it is unlikely the consumer would want to perform such a complicated and time-consuming calibration process. Moreover, with both types of calibration techniques, if the consumer performed the calibration process improperly any results obtained from the range imaging system would be erroneous.
Accordingly, there exists a need for a range camera calibration method and system that is accurate and simple to use. Whatever the merits of the above-mentioned systems and methods, they do not achieve the benefit s of the present invention.
To overcome the limitations in the prior art as described above and other limitations that will become apparent upon reading and understanding the present specification, the present invention includes a method and system for determining a relative position and orientation of a plurality of range cameras using spatial movement. In particular, a path of an object is measured by each range camera in the camera""s local coordinate frame. Thus, the path of the object is observed by each camera but, because each camera has a different view of the object""s path, the object path is reported by each camera in different local coordinate frames.
The present invention determines the relative location of each range camera by converting the object path as measured in each of the local coordinate frames to a common coordinate frame. The common coordinate frame may be, for example, with respect to one of the cameras or with respect to the scene (such as a room-based coordinate system).
In general, the novel method of the present invention includes measuring a path of an object in a scene as observed by each camera, performing matching of points of the path and obtaining transformation parameters (such as an offset distance (xcex94x, xcex94y) and a rotation angle (xcex8)), preferably by solving a system of transformation equations. These transformation parameters are used to determine the relative position of each camera. Moreover, the present invention includes other novel features such a data synchronization feature that uses a time shift between cameras to obtain the transformation parameters. In addition, the present invention includes a unique process that improves the robustness and accuracy of solving the system of transformation equations by using a process that is less sensitive to outlying points. For example, in a preferred implementation the present invention includes using a least median of squares technique to reduce the sensitivity of the solution to points extremely removed from the correct solution. The present invention also includes an interpolation process that interpolates between sampled points if there is no data at a particular instant in time. Further, the present invention includes a system for determining a relative position and orientation of range cameras using spatial movement that incorporates the method of the present invention.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not by the preceding summary or the following detailed description.